For many years the preferred type of microphone for telecom applications (ie Mobile phones) has been electret microphones. This type of microphone is based on the principle of a capacitor which is formed by a movable member that constitutes a membrane of the microphone and another member, eg a so-called back plate of the microphone. One of the members of the microphone, preferably the membrane, is provided with a captured electrical charge also known as an electret layer.
However, in recent years microphones are also provided with the charge by a DC voltage source during operation. These microphones comprise ordinary condenser microphones and MEMS microphones which are on their way to be utilized in telecom applications.
Microphones without an electret layer can be implemented in two different ways. That is, on the one hand, in the conventional way where the parts are manufactured as metal parts for mounting in a case (which normally forms both a capacitor plate of the microphone and a backplate).
On the other hand, in recent years also silicon usually used for integrated circuits have been used for the manufacture of mechanical structures. This technology is usually denoted MEMS. The differences between manufacture of microphones in MEMS technology and conventional technology are mainly the processes involved and the tolerances. I.e. MEMS technology requires clean rooms and silicon technologies. The precision is higher for silicon technologies but so is the cost.
The characteristics of the two types of microphones are that the microphone capacitances are usually smaller and the required bias voltages are smaller. This is due to the fact that microphones implemented in silicon have to be smaller to be able to compete cost-wise. This means that the silicon microphones are optimized differently even though the principle of the two microphone types is the same. I.e. the membrane area of a silicon microphone is typically 1 mm^2, airgap 1-2 um, capacitance 1 pF and bias voltage 10V. And for a conventional microphone it is area=3 mm^3, airgap=10-15 um, capacitance 3 pF and bias voltage 30-40V. Or even larger bias voltage.
As the charge on the microphone capacitor has to be kept constant to maintain proportionality between sound pressure and voltage across the capacitor members, it is important not to introduce any de-charging of the microphone.
Therefore, in order to pick up a microphone signal from the capacitor, amplifiers configured with the primary objective of providing high input resistance are preferred to buffer the capacitor from circuits which are optimized for other objectives. The amplifier connected to pick up the microphone signal is typically denoted a preamplifier or a buffer amplifier or simply a buffer. The preamplifier is typically connected physically very close to the capacitor—within a distance of very few millimetres or fractions of millimetres.
For small-sized microphones only a very limited amount of electrical charge can be stored on one of the microphone members. This furthers the requirement for high input resistance. Consequently, the input resistance of preamplifiers for small-sized microphones has to be extremely high—in the magnitude of Giga ohms. Additionally, the input capacitance of this amplifier has to be very small in order to achieve a fair sensitivity to sound pressure. Telecom microphones with an integrated preamplifier are sold in high volumes and at very low prices. As the cost of an amplifier for a telecom microphone is directly related to the size of the preamplifier chip die, it is important, for the purpose of reducing price, that the preamplifier die is as small as possible.
So, obviously, there is a need for microphone preamplifiers with gain and very low input capacitance, and the lowest possible preamplifier die area. Additionally, low noise is important. Low noise is important as noise can be traded for area—ie if the circuit has low noise and a noise lower than required, this noise level overhead can be traded for lower chip die area and it is thus possible to manufacture the preamplifier at lower cost.
However, since sensitivity is traded for low prise, microphones for telecommunications purpose are less sensitive. From a market perspective, there is a demand for a larger sensitivity of the microphone and preamplifier in combination. So therefore the gain in the preamplifier is to be increased to meet the demand. Additionally, there is a demand for low noise in the audible range. Moreover, in order to ensure a good signal-to-noise ratio while meeting the demand for a relatively large sensitivity, the input capacitance of the preamplifier must be small to avoid an unnecessary signal loss from the microphone (cf. the equivalency of the microphone signal being exposed to a voltage divider constituted by the capacitances).
Since the chip area occupied by the preamplifier must be as small as possible to obtain relatively low cost, the preamplifier must be as small as can be. Therefore, since amplifier configurations known from hearing aids are generally not optimised for chip area to the same extent, these configurations are not applicable. Further, one should bear in mind that buffers or amplifiers applied in hearing aids are not configured to provide such high gain levels as are required for the low-sensitivity microphones used in telecommunication applications. In hearing aids chips more space is required for the same noise performance since buffers are required to avoid overload in hearing aids.
The charge on the microphone capacitor can be provided by a relatively high DC supply voltage or by a manufacturing process where a static charge is captured on one of the capacitor members eg on the membrane which can made from Teflon. The type of microphone where a static charge has been applied is preferred in telecom applications since this type does not require a circuit for supplying the charge by a DC voltage. However, this type has been found shown to lose the charge when exposed to relatively high temperatures. Additionally, such microphones require careful handling and mounting during eg a soldering process which inherently exposes the microphone to high temperatures. When the microphone has lost its static charge, the microphone's ability as a sound transducer is diminished and it will be far too inexpedient to re-establish the charge.
Noise is also an important parameter for telecom microphones. Typically, the predominant noise sources are related to the preamplifier in the microphone. But when switching and/or oscillating circuits are incorporated on the chip die, surrounding/neighbouring circuits/circuit paths with transient signals can become major noise sources. Additionally, when a noisy signal is transmitted on the same path or terminal as another signal, eg the microphone signal, this noisy signal will constitute a major and direct noise source of which the noise influence may be difficult to suppress.
When designing a preamplifier in CMOS technology for a microphone there is normally three noise sources. These sources are noise from a bias resistor, 1/f noise from an input transistor, and white noise from the input transistor. We assume that input transistor noise dominates. Both white noise and 1/f noise can be minimized by optimization of the length and the width of the input transistor(s). This applies for any input stage, eg a single transistor stage or a differential stage. The noise from the bias resistor can also be minimized. If the bias resistor is made very large the noise from the resistor will be high pass filtered and the in-band noise will be very low. This has the effect, though, that the lower bandwidth limit of the amplifier will be very low. This can be a problem as the input of the amplifier will settle at a nominal value only after a very long period of time after power-up. Additionally, signals with intensive low frequency content arising form eg slamming of a door or infra sound in a car can overload the amplifier. Another related problem is small leakage currents originating from mounting of the die inside a microphone module. Such currents will due to the extreme input impedance establish a DC offset. This will reduce the overload margin of the amplifier.